1. Field of the Invention
The present invention relates to a hydraulic hybrid vehicle comprising: a power train including an engine and a transmission; a pump motor; and an accumulator, wherein the pump motor is driven as a motor by hydraulic pressure accumulated in the accumulator to effect travel of the vehicle, to assist the engine, or to start the engine, and the pump motor is driven as a pump by drive power from the engine or by drive power transmitted back from a drive wheel to accumulate pressure in the accumulator.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 8-282324 has made known a hydraulic hybrid vehicle in which an engine is connected to front wheels through a transmission and a pump motor which functions as both a pump and a motor is connected to rear wheels, and, at the time of starting, the pump motor is driven as the motor by hydraulic pressure accumulated in an accumulator and, at the time of braking, the pump motor is driven as the pump by drive power transmitted back from the rear wheels thereby to accumulate pressure in the accumulator.
Japanese Patent Application Laid-open No. 6-107139 has also made known a hydraulic hybrid vehicle in which a drive shaft for transmitting drive power from an engine to a drive wheel is connected to a pump motor through a dog clutch and a gear box, and tooth breakage in the dog clutch is prevented by disengaging the dog clutch when the number of revolutions of the pump motor reaches its maximum permissible number of revolutions, and by engaging the dog clutch when a difference in revolution between the number of revolutions of the pump motor and the number of revolutions of the drive shaft becomes less than a permissible number of revolutions.
Incidentally, when such hydraulic hybrid vehicles include a power train formed of the engine and the transmission, an oil tank and a pump motor for a hydraulic hybrid system are necessary in addition to an oil pan and an oil pump for supplying hydraulic oil or lubricating oil to the power train, and the coexistence of these two hydraulic systems poses a problem of involving an increase in the number of parts, an increase in size of an apparatus, an increase in weight, an increase in cost, and the like.